1. The present invention relates to reaction sintered silicon carbide structures with improved impact strength.
2. Silicon carbide base ceramic materials are used in their sintered and hot pressed form in various high temperature structural applications. In demanding applications, such as vanes and blades for gas turbine engines, the properties required include not only high temperature strength, corrosion and oxidation resistance, but resistance to sporadic impact damage from projectiles. Silicon carbide, like many other ceramic materials, is found in materials tests to have relatively low energy of fracture.
In practice, this limited resistance to impact damage limits the utility of the materials and various approaches have been utilized to try to impart improved properties to ceramic articles. For example, metal fibers and other rod-like structures have been integrated with the ceramic, but this approach has met with only limited success due to the limitations imposed by the properties of the metals. Another approach utilized has been to provide a crushable surface layer on the ceramic. For example, duplex silicon carbide structures comprised of a fully sintered high density substrate in combination with an integral partially sintered and densified silicon carbide surface layer are described in the Reports No. NASA CR-134990 and NASA CR-134921, "Improved Toughness of Silicon Carbide", J. A. Palm, (published by the General Electric Co. for National Aeronautics and Space Administration, Washington, D.C. 1975-1976). A limitation of this duplex approach is that the component surface must be formed to final shape in fabrication, without subsequent machining, elsewise the energy absorbing layer will be removed.
For silicon nitride base ceramics, a porous impact energy absorbing surface layer may be provided by partially nitriding a slurry of silicon powder which is deposited on a component surface, as described in our copending U.S. patent application Ser. No. 054,522, filed July 2, 1979 now U.S. Pat. No. 4,252,860. Silicon nitride is commonly formed by reaction sintering, i.e., heating silicon in a nitrogen atmosphere, but there is not a typical analogous process for forming silicon carbide articles; sintering of the easily formed carbide particles is prevalent.